Performance of a vector velocity estimator

It is a well-known limitation of all commercially available scanners that only the velocity component along the propagation direction of the emitted pulse is measured, when evaluating blood velocities with ultrasound. Proposals for solving this limitation using several transducers or speckle tracking can be found in the literature, but no method with a satisfactory performance has been found that can be used in a commercial implementation. A method for estimation of the velocity vector is presented. Here an oscillation transverse to the ultrasound beam is generated, so that a transverse motion yields a change in the received signals. The method uses two ultrasound beams for sampling the in-phase and quadrature component of the lateral field, and a set of samples (in-phase and quadrature in both time and space) are taken for each pulse-echo line. These four samples are then used in an autocorrelation approach that yields both the axial and the lateral velocity, and thus the velocity vector. The method has the advantage that a standard array transducer and a modified digital beamformer, like those used in modern ultrasound scanners, is sufficient to obtain the information needed. The signal processing preceding the beamforming can be implemented using standard signal processors, and it is robust since the autocorrelation method is used. Measurements are obtained using a point scatterer and a sponge. 250 pulse-echo lines where measured for each object under investigation making it possible to obtain 22 estimates, when using 20 lines with a 50 % overlap in the transverse estimator. The movement of the scatterer was created with a translation stage with a controllable step size. The step size used for the measurement was 40 μm and 250 lines were recorded for angles of 90, 75, 60, and 50 degrees. The results show a bias in the lateral velocity estimate of -18%. The overall standard deviation averaged over all angles was 29.5%.